Traditional radio altimeter systems have been plagued with many limitations. Most significantly, traditional Low Range Altimeter (LRA) installation systems form a closed loop. This close loop is normally formed from transmitting a modulated signal in the 4.3 GHz band on a transmitting antenna and receiving the ground reflected signal via a second receiving antenna. Then, the system mixes the ground reflected signal with the transmitted modulated signal within the receiver to determine the round-trip delay of the signal.
These closed loop systems induce challenges to manufacturers and operators desirous of accurate radio altitude information in a variety of ways. Since each of the signals is vital to proper operation, a high level of isolation is required between the transmit (Tx) antenna and the receive (Rx) antenna. Traditional systems are prone to false altitude detection from leakage (reflections from surrounding vehicle structure between Tx and Rx antenna). Cable routing and shielding is also critical to avoid leakage. This labor intensive routing and shielding requires continuous isolation between coaxial feed lines greater than 120 dB.
Traditional systems further require a calibrated delay for zero feet altitude (defined as the Aircraft Installation Delay (AID)). Specific lengths of coaxial cable must be used to ensure the correct AID. This requires the Line Replaceable Unit (LRU) to be located close to the antenna (approximately 12 to 15 feet).
Consequently, a need remains for an alternative to large form factor radio altimeter systems requiring multiple antennas and labor intensive cable routing and shielding.